1. Field of the Invention
This invention relates to a novel drug-permeable polymeric wall adapted to release a drug at a controlled rate from a drug-containing reservoir to a body environment and to novel drug delivery devices including such a wall. It also relates to methods for increasing the permeability of polymers used to make such walls and methods for modifying the rate of drug release through such walls.
2. Description of Prior Art
Prior art drug delivery devices comprised of a drug dispersed in a solid matrix permeable to passage of the drug and surrounded by a polymer membrane, also permeable to passage of the drug but at a lower rate than through the matrix, have proven themselves capable of zero order drug release and represent a substantial improvement over previously proposed drug delivery devices. The drug release rate in these devices is controlled by using certain wall forming materials for releasing the drug, and the release rate for the drug is proportional to its diffusion coefficient in the wall and to the solubility of the drug in the wall. In making these drug delivery devices available to the art, these parameters are developed for each drug, and it has now been found that this can require a large number of wall forming materials for different drugs. This often adds considerable complexity to the design of drug delivery devices.
It is also known to the art to incorporate drug into certain types of liquid carriers, usually in microcapsule formulations, for example, as in U.S. Pat. No. 3,464,413. However, these microcapsules are not designed for the controlled release of drug for a prolonged period of time by using drug release rate controlling wall materials. The microcapsules are frequently crushable, and they merely function as drug carriers supplying their drug in bulk, and not in controlled amounts by rupture of the microcapsules. These types of capsules are not suitable for releasing drug at a controlled rate for a prolonged period of time.
The prior art also discloses incorporating conventional plasticizers in polymer walls of drug delivery devices (U.S. Pat. Nos. 3,630,200 and 3,538,214) to increase the flexibility of such walls.